This invention relates in general to electronic packaging technology and in particular, to apparatus and method for testing the calibration of electronic package lead inspection system.
With increasing pressure to reduce electronic package size such as the size of packages for semiconductor dies, the trend in packaging technology is to reduce the size of the leads in order to increase pin counts. As pin counts of devices increase, characteristics such as lead coplanarity and lead-to-trace registration and other parameters become critical for the printed circuit board (PCB) assemblers. Poor package coplanarity will result in open connections between package pins and PCB traces, and poor lateral registration can result in open, shorts and misconnections. Due to customer demand, the integrated circuit (IC) manufacturers have to comply closely to customer specified package dimension parameters. No entirely satisfactory solution has been found to assure compliance with such specifications. Some compliance systems involve manual inspection using optical comparators or shadowgraphs while others are automated.
After the leads of the package have been trimmed or formed by an IC manufacturer, the leads are inspected by a lead inspection system. They are then packaged and shipped, if necessary, to a test site where they are further processed. At the test site, the leads are again inspected by a lead inspection system to ensure that the various important parameters of the leads still comply with specifications. After such further processing and testing, they are again packed and shipped to the PCB assemblers. Before the packages are mounted onto PCBs, the leads of the packages are again inspected by lead inspection systems. The above-described processing and handling of the package are illustrated in FIG. 1. FIGS. 1-4 as well as much of the discussion in the background of the invention have been taken from an article entitled "3-D Scanner for Quad Flat Package Measurement and Inspection," by Scott A. Erjavic and Sullivan Chen; the article will be published in the 1991 Proceedings of Surface Mount International. The article is incorporated herein in its entirety by reference.
Plastic leaded chip carrier type packages are normally inspected using two-dimensional imaging inspection systems. Two-dimensional inspection systems typically evaluate component geometries either by a back-lit profile or a front- or oblique-lit reflection. Some applications utilize a combination of both. Backlit imaging systems rely upon a sharp contrast being seen by the detector at the edge of the feature being inspected, i.e., the edge of the package lead. CCD arrays in the detector analyze the feature boundary for gray scaled transitions to/from black/white on adjacent array pixels. Absolute occurrence of objects on the array defines positional information with which the gray scale information is associated. In situations where pixel densities are high relative to the feature geometries, this method can provide very accurate positional information. But with a fixed density array, the ability to migrate to larger package sizes and higher lead counts and lead pitches is limited.
Quad flat pack (QFP) type packages are normally inspected using three-dimensional lead inspection systems.
FIG. 2 illustrates a three-dimensional laser scanner using a laser source. As shown in FIG. 2, the light reflected from a lead of the package is focused by a lens towards a detector. The position of the detector that senses the reflection will indicate the Z-axis height information. The technique for measurement of X-axis and Y-axis location of the lead is well known. Many lead inspection systems employ different algorithms for computing different parameters that are important when the PCB assembler aligns and attaches the leads of the package to conductive traces on the PCB. Such algorithms are implemented by means of electronic systems.
Before the leads of a package are scanned as illustrated in FIG. 2, the package must first be removed from a shipment container from a test site as shown in FIG. 1 and delivered to the lead inspection site for scanning. When the various parameters important to the PCB assembler are calculated from the detector readings, various assumptions are made on the positions of the lead in relation to the laser and the detectors. Thus if the system for placing the package places the package at a location relative to the laser and detectors slightly different than what is assumed, the readings of the detectors will provide an inaccurate measurement of the parameters. The components in the electronics system for performing a different algorithm for calculating the parameters may also experience drift. All such factors may introduce slight variations or even large errors in the parameter measurements. A functional block diagram of a conventional laser inspection system is shown in FIG. 3.
To assist in the calibration of the scanner so that the above problems are alleviated, conventional scanner systems such as scanner 18 frequently employ a precision tool 22 of known NIST dimensions where tool 22 is at a known location relative to a 3-D sensor 20. In most cases, this means that the precision tool 22 is rigidly mounted onto the sensor support 24 which also supports a tray 26. Thus before sensor 20 is used to scan the leads in packages held in tray 26, sensor 20 is first initialized by scanning tool 22. Since tool 22 is of known NIST dimensions and at a location precisely known relative to sensor 20, the sensor can be calibrated relative to the scanner readings on the tool. Thereafter, when sensor 20 is used for scanning the leads or packages in tray 26, the sensor is sensing by reference to the tool of known NIST dimensions and at least some of the uncertainty in interpreting scanner readings will be reduced.
At the test site shown in FIG. 1, before the electronic packages are packaged and shipped to the PCB assembler, a small number (such as 35) out of a lot of packages (such as 2,000) are scanned using the scanner of FIGS. 2 and 3 to provide plots of the important parameters. These plots are compared to preset upper and lower limits in graphs such as those shown in FIG. 4 so that the scanner operator can ascertain whether the important parameters of the packages sampled are within the upper and lower limits. When such parameters sampled are within the prescribed limits, it is assumed that the same parameters for the whole lot are also within the upper and lower limits and the lot is then packaged and shipped to PCB assemblers. However, if plots of the parameters of some of the packages sampled fall outside the limits, it is then necessary to determine whether the leads of the packages are out of alignment or whether the lead inspection system or the package transport system are out of calibration.
Even with the aid of the precision tool described above, the large number of variables involved in the lead transport and lead scanning systems is such that it is still difficult to ascertain whether the lead inspection system or the package transport system itself is in error or whether the leads of the packages inspected are out of alignment. This determination is critical since misaligned packages should not be shipped to the PCB board assembler without any correction measures being taken. Furthermore, since lead inspection is performed at three different locations as shown in FIG. 1, different types of scanners used at the three sites or differences in their calibration make it even more difficult to identify the source of problems should they arise.
It is therefore desirable to provide a system for testing the calibration of lead inspection systems to further alleviate the above-described problems.